Shelf



FIG. 1 is a top and front perspective view of a shelf showing my new design, the broken-line disclosure of a portion of a wall being for illustrative purposes only and forming no part of the claimed design;

FIG. 2 is a reduced rear elevational view thereof; and,

FIG. 3 is a reduced front elevational view thereof. 

The ornamental design for a shelf, as shown and described. 